Method for producing high-purity hydrochloric acid

ABSTRACT

This invention relates to a novel method, which can be carried out according to industrial standards, for producing high-purity hydrochloric acid with a very low particle content, for use in the production of semiconductors.

The present invention relates to a novel industrial process forproducing high purity, low particle hydrochloric acid for use insemiconductor production.

The hydrogen chloride gas produced by chlorine electrolysis andsubsequent burning of chlorine and hydrogen customarily containsimpurities which is [sic] very difficult if not impossible to removefrom the gas itself. Such impurities include for example arsenic,bromine and volatile organic impurities. Such impurities are difficultto remove even by passing the hydrogen chloride gas into water andsubsequently subjecting the resulting hydrochloric acid to an adiabaticdistillation.

It is known to produce hydrochloric acid in more or less high purityaccording to the following different methods:

1. Distillation of 20% hydrochloric acid

2. Distillation of a hydrochloric acid having a hydrogen chloridecontent of >20% and subsequent absorption of the released hydrogenchloride in the 20% hydrochloric acid distillate

3. Introducing hydrogen chloride gas from compressed gas bottles or apressurized pipework system conveying hydrogen chloride into a receiverpartly filled with water and provided with a cooling means

4. Subboiling distillation

These methods (1-4), however, do not permit production of a hydrochloricacid in the requisite purity (methods 1, 2, 3) or requisiteconcentration (methods 1 and 4).

A method suitable for producing small amounts of high purityhydrochloric acid has hitherto been carried out as follows: 37%hydrochloric acid is heated to expel hydrogen chloride gas. The hydrogenchloride gas is passed through a small retention column to removedroplets and subsequently into a receiver filled with ultrapure water.However, this method is only suitable for producing small quantities ofhydrochloric acid, i.e. 10-20 t/year, in a batch operation, since thepressure in the plant will change constantly with increasing fill level.The latter renders continuous operation in the vaporization part of theplant impossible or would necessitate a substantial and complicatedtechnical effort.

It is an object of the present invention to provide a simple andinexpensive way of producing on an industrial scale for use in thesemiconductor industry a hydrochloric acid having an extremely low levelof cationic, anionic and particulate impurities. In accordance with theintended use, the hydrochloric acid produced should have a hydrochloricacid content of 35-38%.

This object is achieved by a process for producing high purity, lowparticle hydrochloric acid for use in the semiconductor industry,characterized in that

a) hydrogen chloride gas is expelled from a hydrochloric acid having ahydrogen chloride content of more than 21% by heating,

b) the hydrogen chloride gas is passed through a retention column and ademister, each made of fluorinated or perfluorinated polyolefin, and

c) subsequently dissolved in ultrapure water in an absorption column toform hydrochloric acid.

For concentrating, the invention makes it possible for the hydrogenchloride solution obtained to be withdrawn, cooled and recycled backinto the column.

In one embodiment of the invention, HCl-containing water vapour escapingat the top of the absorption column is passed into a packed column andcondensed in a downstream column.

The concentration of the hydrogen chloride solution may be determined byconductivity measurement. In a continuous process, this can be donecontinuously.

The invention provides in particular such a process as operates at lowflow resistance, so that it may be carried out at a pressure in therange from atmospheric pressure to a small overpressure of 500 mm ofhydrostatic head, preferably <200 mm of hydrostatic head, and constantconditions.

In one embodiment of the process according to the invention, thehydrogen chloride solution is recycled into the absorption column toproduce a hydrochloric acid having a concentration of >32%.

In another embodiment of the process according to the invention, thehydrogen chloride solution is recycled into the absorption column toproduce a hydrochloric acid having a concentration of 35-38%.

In one version of the process, the hydrochloric acid solution isadjusted to the desired concentration by addition of ultrapure water.

If necessary, any particulate impurities present or formed in the highpurity hydrochloric acid may be removed prior to filling in suitabletransit containers by filtration using a 1- to 3-stage filter unit.

In particular, in the case of multistep filtration, the filtration canbe effected in succession using filters of decreasing pore size between1.0 and 0.05 μm.

As stated earlier, combining a vaporization plant with an adiabaticabsorption column provides high purity hydrochloric acid having aconcentration of 35 to 38% on an industrial scale that meets therequirements for use in the semiconductor industry.

This advantageous result can be obtained by using plant components andfeed vessels made of fluorinated or perfluorinated polyolefin materialsof construction.

For concentrating, the solution initially obtained in the upper columnsection (8), which has a hydrogen chloride content of not more than 32%,is cooled in a heat exchanger (9) and then fed to the 2nd column section(10) disposed underneath the upper column section (8) and the cooler(9).

To preclude the product being contaminated via the top of the uppercolumn section (8), through which the water vapour containing HCl gasresidues is withdrawn, the escaping gas (HCl and water vapour) is leddownwards through a safety column (13) and then condensed in acondenser.

By recycling the hydrogen chloride solution into the lower section ofthe absorption column (10), it is possible to further increase theconcentration of the acid and hence produce a hydrochloric acid having aconcentration of more than 32%.

In one version of the process, initially a hydrochloric acid having ahydrogen chloride content above the desired concentration is producedand then by controlled addition of ultrapure water produced in aspecific manner to the desired concentration within a very narrowtolerance range (±0.2%) by controlled addition of ultrapure water.

To carry out the process of the invention, a vaporization plant is usedto expel hydrogen chloride gas from a hydrochloric acid having ahydrogen chloride content of >20%, preferably having a hydrogen chloridecontent of 22-40%, by heating at atmospheric pressure or only slightlyelevated pressure (0−max. 500 mm hydrostatic head).

Troublesome impurities remain behind in the approximately 20% residualacid to obtain a high purity hydrogen chloride gas.

The hydrogen chloride stream is fed under atmospheric or only slightlysuperatmospheric pressure initially through a retention column to removeliquid fractions, for example acid traces in the form of liquiddroplets, and through a downstream demister under adiabatic conditionsinto the absorption column and absorbed therein in ultrapure water. Toobtain a hydrochloric acid having a concentration of >32%, preferably 35to 40%, the hydrogen chloride solution descending from the upper columnsection (8) is cooled and then fed back into the lower column section(10) to concentrate the acid to the desired hydrogen chloride content.

As mentioned above, it is particularly advantageous to use retentioncolumns and demisters made using a fluorinated or perfluorinatedpolyethylene as material of construction.

The top of the absorption column is supplied with high purity water. Theamount of water added is a simple way of controlling the concentrationof the high purity hydrochloric acid obtained, and the concentration ofthe descending acid and hence also the amount of ultrapure water to beadded can be monitored by a continuous conductivity measurement at theexit from the 2nd column section.

Tests have shown that the plant system will provide trouble-freeprocessing only when the flow resistances of all components are suchthat the flow resistance in the overall system is so low that a pressureof 500 mm hydrostatic head, preferably <200 mm hydrostatic head, is notexceeded and maintained virtually constantly in the vaporizationsection. In other words, the plant can be operated virtually atatmospheric pressure under constant conditions.

The water vapour which escapes at the top of the absorption column andwhich may still contain very small amounts of HCl gas is protected fromthe external atmosphere by a packed column (13). The water vapour iscondensed in a downstream condenser. To monitor the column system, theconcentration of the hydrogen chloride is continuously recorded byconductivity measurement.

To be able to comply with the narrow concentration tolerances stipulatedin semiconductor fabrication, it is advantageous for the concentrationof the acid exiting from the absorption column to be somewhat higherthan required. A downstream buffer cycle is then used to adjust thedesired concentration within the requisite tolerance limits by regulatedfurther addition of ultrapure water. In this section of the plant tooall components coming into contact with the product are made offluorinated or perfluorinated polyolefin materials of construction.

Tests on different materials of construction have shown that the choiceof suitable materials of construction in particular, not only for theplant but also for the storage and transit systems subsequently used,makes it possible to avoid contamination due to the materials ofconstruction. More particularly, this prevents contamination not only ofthe pure hydrogen chloride stream but also of the hydrochloric acid.This applies very particularly to the column packings used in theretention column and in the absorption column and to the demister, whoserelease of ionic contaminants on contact with the product has to be solow that the purity required for the product can be achieved.

The product obtained is preferably collected in storage vessels linedwith fluorinated or perfluorinated polyolefin materials. After qualitycontrol, hydrochloric acid is released and filled via a clean couplingbox into transit containers lined with fluorinated or perfluorinatedpolyolefin materials or directly into suitable small containers whichlike the transit containers conform to the DIN ISO regulations.

If necessary, the hydrochloric acid is filtered in a 2- to 3-stagefiltration unit before filling into suitable storage vessels tosuccessively remove particles having an average diameter of >1 μm, >0.2μm and >0.1-0.05 μm.

The sections of the plant which come into contact with the hydrochloricacid or the hydrogen chloride gas or else with the ultrapure water, butalso the storage vessels, are constructed of materials which, under thegiven conditions, release neither ionic nor particulate contaminants.Such materials are materials of construction based on polyolefin,preferably fluorinated or perfluorinated polyolefins such as PVDF, PFA,PTFE and PTFE-TFM. PVDF, PFA and PTFE-TFM are particularly useful notonly from qualitative but also from economic aspects.

In contrast to conventional plants for obtaining high purity and lowparticle hydrochloric acid for use in the semiconductor industry, theinventive plant described herein makes it possible to produce, on alarge scale and selectively, hydrochloric acid products having a verylow hydrogen chloride content but also having a hydrogen chloridecontent of up to 40%.

The concentrations of possible cationic contaminants in the hydrochloricacid obtained according to the invention are so low that they are eitherbeyond detection by the analytical methods for hydrochloric acidavailable at present or are very close to the currently achievable limitof detection, i.e. they are below the detection limit of <0.05 ppb. Thelevel of anionic contaminants is likewise in virtually all cases belowthe analytical detection limit currently achievable in hydrochloricacid.

More particularly, the process described makes it possible to lower thebromide fraction customarily in hydrochloric acid to a level of <1 ppm.

By way of example, Table 1 reports analytical values determined in ahigh purity hydrochloric acid produced according to the claimed process.

TABLE 1 Analytical values measured in a plant designed according to theprocess described and having a capacity of 350 kg of high purity 36%hydrochloric acid Values (if no other dimens. is listed) in ppb Assay %35-37 Reducing Iodine <500 Free Chlorine <100 Ammonium NH₄ <200 BromideBr <1000 Nitrate NO₃ <100 Phosphate PO₄ <50 Sulfate SO₄ <50 Resid. ofignition <500 as sulfate Aluminium Al <0.1 Antimony Sb <0.1 Arsenic As<0.05 Barium Ba <0.05 Beryllium Be <0.05 Bismuth Bi <0.05 Boron B <0.1Cadmium Cd <0.05 Calcium Ca <0.1 Chromium Cr <0.05 Cobalt Co <0.05Copper Cu <0.05 Gallium Ga <0.05 Germanium Ge <0.05 Gold Au <0.05 IndiumIn <0.05 Iron Fe <0.1 Lead Pb <0.05 Lithium Li <0.05 Magnesium Mg <0.1Manganese Mn <0.05 Molybdenum Mo <0.05 Nickel Ni <0.05 Niobium Nb <0.05Palladium Pd — Platinum Pt — Potassium K <0.05 Silicon Si <5 Silver Ag<0.05 Sodium Na <0.1 Strontium Sr <0.05 Tantalum Ta <0.05 Thallium Tl<0.05 Tin Sn <0.1 Titanium Ti <0.1 Vanadium V <0.05 Zinc Zn <0.05Zirconium Zr <0.05 Particles >0.2 μm N/ml <100

FIG. 1 shows the inventive layout of a plant for carrying out theinventive process for producing high purity, low particle hydrochloricacid for use in the semiconductor industry. The plant includes thefollowing elements:

1. Retention column

2. Hydrochloric acid feed

3. Vaporizer

4. Outlet for continuous removal of residual acid

5. Demister

6. Hydrogen chloride gas line

7. Ultrapure water feed

8. Upper section of absorption column

9. Intermediate cooling

10. Lower section of absorption column

11. Cooling

12. Outlet for obtaining the end product

13. Safety column

14. Condensation.

What is claimed is:
 1. A process for producing a high purityhydrochloric acid for use in the semiconductor industry, comprising: a)expelling hydrogen chloride gas from a hydrochloric acid having ahydrogen chloride content of more than 21% by heating, b) passing thehydrogen chloride gas initially at or slightly above atmosphericpressure through a retention column to remove liquid fractions and ademister operating under adiabatic condition, each comprisingfluorinated or perfluorinated polyolefin, and c) passing the gasdirectly into an absorption column wherein the gas is subsequentlydissolved in an ultrapure water in the absorption column to formhydrochloric acid and a hydrogen chloride solution descending from anupper column section is cooled and then fed back into a lower columnsection to obtain a hydrochloric acid having a concentration >32%.
 2. Aprocess according to claim 1, further comprising passing HCl-containingwater vapor escaping at the top of the absorption column into a packedcolumn and condensed condensing in a downstream column.
 3. A processaccording to claim 1, further comprising determining the concentrationof the hydrogen chloride solution by conductivity measurement.
 4. Aprocess according to claim 1, wherein the process is carried out at apressure in the range from atmospheric pressure to an overpressure of500 mm of hydrostatic head, and constant conditions.
 5. A processaccording to claim 1, wherein the hydrogen chloride solution is recycledinto an absorption column to produce a hydrochloric acid having aconcentration of >32%.
 6. A process according to claim 1, wherein thehydrogen chloride solution is recycled into the absorption column toproduce a hydrochloric acid having a concentration of 35-38%.
 7. Aprocess according to claim 1, wherein the hydrochloric acid solution isadjusted to the desired concentration by addition of the ultrapurewater.
 8. A process according to claim 1, further comprising removingformed particles prior to filling in a suitable transit container byfiltration using a 2- to 3-stage filter unit.
 9. A process according toclaim 8, wherein filtration is effected using in succession filters ofdecreasing pore size of 1.0-0.05 μm.
 10. A process according to claim 1,wherein the process is conducted at a pressure of less than 200 mm ofhydrostatic head.
 11. A process according to claim 1, wherein theabsorption column comprises fluorinated or perfluorinated polyolefin.12. A process according to claim 1, wherein the fluorinated orperfluorinated polyolefin is polyvinylidene fluoride, perfluoroalkoxyresin, polytetrafluoroethylene or PTFE-TFM.
 13. A process according toclaim 1, wherein the hydrochloric acid from c) comprises: Values in ppbif no other units are listed Reducing Iodine <500 Free Chlorine <100Ammonium NH₄ <200 Bromide Br <1000 Nitrate NO₃ <100 Phosphate PO₄ <50Sulfate SO₄ <50 Resid. of ignition as sulfate <500 Aluminium Al <0.1Antimony Sb <0.1 Arsenic As <0.05 Barium Ba <0.05 Beryllium Be <0.05Bismuth Bi <0.05 Boron B <0.1 Cadmium Cd <0.05 Calcium Ca <0.1 ChromiumCr <0.05 Cobalt Co <0.05 Copper Cu <0.05 Gallium Ga <0.05 Germanium Ge<0.05 Gold Au <0.05 Indium In <0.05 Iron Fe <0.1 Lead Pb <0.05 LithiumLi <0.05 Magnesium Mg <0.1 Manganese Mn <0.05 Molybdenum Mo <0.05 NickelNi <0.05 Niobium Nb <0.05 Palladium Pd — Platinum Pt — Potassium K <0.05Silicon Si <5 Silver Ag <0.05 Sodium Na <0.1 Strontium Sr <0.05 TantalumTa <0.05 Thallium Tl <0.05 Tin Sn <0.1 Titanium Ti <0.1 Vanadium V <0.05Zinc Zn <0.05 Zirconium Zr <0.05 Particles >0.2 μm N/ml <100.


14. A process according to claim 1, wherein the hydrochloric acid fromc) comprises less than 0.05 ppb for each of As, Ba, Be, Bi, Cd, Cr, Co,Cu, Ga, Ge, Au, In, Pb, Li, Mn, Mo, Ni, Nb, K, Ag, Sr, Ta, Tl, V, Zn,and Zr.
 15. A process according to claim 8, wherein the filtrationoccurs after c).
 16. A process according to claim 1, wherein theretention column removes liquid droplets of HCl.
 17. A process accordingto claim 1, wherein the process is conducted at 0-500 mm of hydrostatichead.